Proton Affinity and Protonation Sites of Aniline. Energetic Behavior and Density Functional Reactivity Indices

Abstract

High-level theoretical methods based on both density functional and Hartree−Fock and Moller−Plesset theories have been employed in order to give better insight in the longstanding question concerning the protonation site of gaseous aniline. From the thermodynamic point of view, nitrogen and para ring carbon appear to be the preferred protonation sites. Density functional and MP4 computations show that the protonation on para ring carbon atom is the slightly favored process. The orbital Fukui indices analysis does not resolve unequivocally the dilemma of preferred protonation site attributing to para ring carbon and nitrogen atoms the same probability to work as nucleophilic centers. The absolute hardness values of protonated forms of aniline do not follow the maximum hardness principle. Proton affinity values closer to the experimental counterparts are obtained when the G2(MP2), B3LYP, BP, and MP4 levels of theory are used.